The few people who are experienced enough to develop in CD-i language will come across the rare and scanned documents about the CD-i hardware; for instance Microware's "CD-I System Libraries Technical Reference Manual". CD-i member TwBurn, who is known for developing the USB2CDi adapter and now developing the first new CD-i Homebrew game 'Nobelia' - added a massive undertaking to convert "the CD-i System Libraries Technical Reference Manual (added to his Github). TwBurn: "This document is a manual conversion from the cdisys.pdf, which was accessed from ICDIA. Notes by TwBurn of this document are listed in quote sections like this one. There's probably still a lot of errors in there due to OCR/whatever, especially in the links. I did add a few of my own notes here and there. It's funny to see a few different styles of writing throughout the document."
An interesting thought if this could be the start of CD-i Base: The CD-i Base library was "going to be a collection of library functions that can be used as a base for building a CD-i application. It would ultimately contain functions to access all the commonly-used functions of the CD-i platform and would do so in a way fully conformant with the Green Book. The CD-i Base library would be programmed by CD-i Fan, drawing on his extensive experience in programming the CD-i system. The CD-i Base library was going to be released under the LGPL and sources would be available from the very beginning."
However, both TwBurn and cdifan explain this idea is still far off from happening. "For me work on that is still quite a bit away, I've only touched on a
very small part of CD-i development so far. In order to make it easier
for others there's still quite a way to go, with some big hurdles that
need to be taken." - According to TwBurn.
cdifan adds: "Making gcc-os9-cross working would be a separate project, as there are [currently] no working modern compilers. There is a sort-of working gcc-os9-cross on my HD. I just haven't spent the time to fix a few bugs and nicely release it (on github I think). I had intended CD-i Base to be compatible both with Microware C 3.2 and ANSI C. And if you have gcc, it would automatically be C++ as well."
[Thanks, TwBurn, cdifan, stovent]